Solid tantalum capacitors use as anodes porous pellets which are compressed from fine tantalum and niobium powder. The pellets are first compressed and then they are sintered in a vacuum. The porous structure of the anode provides a high surface area and thereby high specific charge (CV) of the finished capacitors (where CV is a product of capacitance and rated voltage). Generally tantalum pellets contain a lead wire made of the same metal as the compressed powder. The lead wire is attached to the pellet either by welding after the pellet has been sintered, or by embedding in the pellet before the pellet is pressed and sintered.
A major trend of modern tantalum capacitors is reducing of the efficient radius of the powder particles so as to increase the pellet surface and thereby improve the capacitor CV per unit of weight or volume. Usages of very fine tantalum powder requires changing of the pelleting conditions. As the efficient particle radius becomes small the pellet density after pressing (green density) as well as the sintering temperature must be reduced to limit pellet shrinkage during sintering. Reducing of the green density and the sintering temperature causes weakening of the junction between the lead and the pellet for both embedded type pellets and welded type pellets. This causes capacitor rejects at manufacturing and failures at testing. The weakening of the junction between the lead and the pellet is one of the major limitations for usage of very fine tantalum with efficient particle radius of about one micron.
FIGS. 1 and 2 show a prior art capacitor 10 having a lead wire 14 welded to a tantalum pellet 12.
During the welding process the lead wire 14 is pressed against the upper surface of the pellet 12 and heated by current pulse. For low density pellets sintered at low temperature this combination causes cracks of the pellet around welding zone in the area indicated by the numeral 18. The resulting weld 16 may be weakened as a result of the fractured area 18. This is particularly true for tantalum and niobium powder with a particle radius of about one micron or less.
Therefore, a primary object of the invention is the provision of an improved capacitor pellet and lead assembly and method for making same. A further object of the present invention is the provision of an improved capacitor having a lead wire welded to a pellet of low density and small particle size.
A further object of the present invention is the provision of a capacitor having a low density pellet of small particle size which exhibits good AC characteristics.
A further object of the present invention is the provision of a method for welding the lead wire to the pellet which minimizes the fracturing of the pellet during the welding operation.
A further object of the present invention is the provision of a capacitor and method for making same which is efficient in operation, durable in use, and includes high performance characteristics for small particle pellets.